Many substances absorb ultra violet or visible light due to their chemical composition. The absorption of light by substances has been used as the basis for detecting the presence of, and for measuring the concentration of, such substances for many years. The concentration of the substance can be determined by use of the Beer Lambert Law:A=Ebc where A is absorbance,E is the molar absorbtivity with units of L mol-1 cm-1,b is the path length of the sample defined in cm; andc is the concentration of the compound in solution, expressed in mol L-1
The Emax represents the maximum absorption of a substance at a given wavelength.
An absorption monitor system typically comprises a light source for providing light of one or more wavelengths to a fluid sample and an optical detection system for detecting any interaction between the light and the sample. The fluid sample may either be a static sample in a suitable container provided with windows for the optical absorbance measurement or the fluid sample is provided as a fluid flow in a flow cell, as will be disclosed in more detail below. Still further, the monitor system may be of immersion type, wherein optical components of the system are immersed into the fluid sample and are arranged to define an optical detection path there between.
One example of an absorption monitor system is a flow cell UV absorption monitor system that is utilized to measure different absorbance of light at various wavelengths in chromatography systems when separated molecules are eluted from the columns. However, flow cell UV absorption monitor systems may be used to selectively detect samples in a large range of applications other than chromatography systems, such as production fluid process systems, laboratory systems, analytical systems, etc. Monitor systems of this type usually include a light source, a flow cell and a light detector. Ideally, the components of the monitor system are selected and designed to ensure a signal-to-noise ratio with minimal drift and refractive index sensitivity. The light source is typically comprised of a mercury-lamp, a deuterium lamp or a xenon flash lamp. However, in recent years monitor systems using a UV-LED (Light Emitting Diode capable of emitting light in the UV range) as light source have been developed. One example of an UV-LED based monitor system is disclosed in U.S. Pat. No. 8,068,227.
In order to achieve a signal-to-noise ratio with minimal drift and refractive index sensitivity according to above, monitor systems of this type are regularly provided with a reference light detector for registering fluctuations and/or drift in light intensity output from the light source. The output from the reference light detector is used to reduce the influence of such fluctuations etc. by normalization or the like of the output from the monitor system.
As is disclosed in U.S. Pat. No. 8,068,227, UV-LED based monitor systems may be more compact and less complex compared to monitor systems based on other light sources. Therefore, UV-LED based monitor systems are generally less expensive to manufacture. However, there is still a need for less complex and less expensive monitor systems.